Syllabic compandors with pilot control are well known in the prior art.
U.S. Pat. No. 3,895,322 issued July 15, 1975 to Stewart et al discloses a compandor with a pilot frequency outside the signal band, and hence does not anticipate our invention. U.S. Pat. No. 2,986,712 issued May 30, 1951 to Hochrath discloses a compandor with a pilot frequency within the signal band but blocked from the expander output by a filter, and hence does not anticipate our invention.
Syllabic compandors of the prior art, not using pilot control, and ring modulators, are described in "Transmission Circuits for Communications,", Bell Telephone Laboratories 1971, pages 677 to 684 inclusive and pages 125 to 128 inclusive. "Reference Data for Radio Engineers", Howard W. Sams 1969, pages 21-14, 30-30 and 30-31 describes sampling theory and compandors. H. S. Black "Modulation Theory", New York 1953, pages 37 to 57 inclusive, describes sampling theory.
We have no knowledge of prior patent art, publications or apparatus which are relevant to our invention.